Second-encounter social investigative behaviors and the impact of MK-801. After the first-encounter social interaction described in Figure 1, mice were returned to their home cage for 30 min. Second-encounter testing began when a test mouse was placed in the same testing arena as before, and a companion mouse was also placed in the arena at the same time. The companion mouse was either the same as in the first encounter (“familiar”), or different (“novel”). Animals were allowed to move and interact freely, and were videotaped for 5 min. The video was analyzed offline for various animal behaviors of the test mouse only. Data are shown as mean ± SEM (n = 12 – 13). Open bars: first-encounter behaviors. Closed bars: second-encounter behaviors with a familiar companion mouse. Hatched bars: second-encounter behaviors with a novel companion mouse. *, significant difference (p < 0.05) between the first- and the second-encounter behaviors (Wilcoxon test). Top row: individual-oriented behaviors; bottom row: social behaviors.